Structures as shown in FIGS. 1 and 2 are conventionally known in common as microphone apparatuses.
A microphone apparatus 100 shown in FIG. 1(A) has a microphone unit “M” mounted on a distal end of a handle “H” and a porous windshield “W” made of urethane foam or the like covering the microphone unit “M”. As shown in an acoustic equivalent circuit in FIG. 1(B), the windshield “W” serves an acoustic function to be an acoustic resistance for the microphone unit “M”. Accordingly, by changing the direction of wind with the windshield “W”, the microphone apparatus 100 shown in FIG. 1(A) is capable of reducing the occurrence of noise due to the microphone unit “M” catching wind noise (wind force noise). Since the windshield “W” works as the acoustic resistance on the acoustic equivalent circuit as the above, reducing the noise to a large degree, however, means increasing the acoustic resistance, thereby relatively lowering sensitivity of a microphone. That is, the ratio of speech signal to noise (SN ratio) is unchanged.
A microphone apparatus 200 shown in FIG. 2 has a structure in which microphone units “M”, “M” are mounted on both ends of a handle “H” and wired in an electrically reversed phase in order to reduce noise. For the microphone apparatus 200, there must be used two microphone units “M” each with exactly the same frequency characteristic and phase characteristic. If the frequency characteristics differ even slightly while the phase characteristics are identical, an electrical output includes a noise output by the difference in sensitivities of the two microphone units “M”. If the phase characteristics differ while the frequency characteristics are identical, the electrical output includes a noise output by the difference in phases of the two microphone units “M”.
Although the microphone apparatus 200 shown in FIG. 2 is superior in theory, there is a need to manufacture homogeneous microphone units “M” with no variations in characteristics, which brings high cost. When the microphone apparatus 200 is used in a narrow space that influences the frequency characteristic or phase characteristic of one of the two microphone units “M”, the effect of noise reduction cannot be obtained.
FIG. 3 is a schematic cross sectional view of a microphone unit “M” with a common directional characteristic. The microphone unit “M” has a structure where sound waves are input from sound openings “So” provided on back and forth sides of an inner diaphragm “d” (upper and lower sides in FIG. 3). When sound waves with the same phase are input from two sound openings “So” to the diaphragm “d”, a superior effect of noise reduction will be brought out. The microphone unit “M” also has a structure capable of reducing noise due to a sound pressure from the side of the microphone unit “M” as shown in an arrow. The effect of noise reduction is however not brought out for use in a narrow space that gives an acoustic influence to the two sound openings “So”.
As shown in FIG. 4, in a typical noise distribution, low-frequency components account for most part, and the higher the frequency goes the more the attenuation occurs. The ordinate axis of FIG. 4 represents sound pressure, which is the level of noise, and the abscissa axis represents frequency. In order to recreate a noise distribution in a narrow space, the microphone unit “M” is actually arranged within a full-face type helmet 50 as shown in FIG. 5 so that a sound opening “So” faces the mouth of a wearer 60 of the helmet 50, and then wind is blown into the helmet 50 by a hair drier 70. Thus, a noise distribution shown in FIG. 6 is measured.
In FIG. 6, “A” represents a frequency characteristic of a measurement result with the microphone unit “M” alone, and “B” represents a frequency characteristic of a measurement result with the microphone unit “M” covered by a windshield made of urethane foam. From FIG. 6, it is understood that the windshield does not work effectively for wind noise.
Now, under an environment with large noise from outside, it is common to put the microphone unit “M” closer to a sound source such as mouth in order to prevent noise from inputting to the microphone unit “M”. In this case, the volume of sound input to the microphone unit “M” becomes excessive, thereby generating a distortion of output. As a countermeasure, an amplifier is used in an electrical circuit to perform an appropriate correction of sensitivity or a large acoustic resistance is provided for preventing the distortion. This attenuates a speech signal and noise relatively, and consequently the SN ratio does not change at all.
Patent document 1 (Japanese Utility Model Laid Open H5-18188) discloses a wind noise preventing type microphone apparatus that has a cylindrical case with a bottom which houses a microphone unit held by a microphone holder made of an elastic material, and has a foamed body with a predetermined width, which is sandwiched between a protector with a sound opening at a center portion thereof and an equalizer with a sound opening at an eccentric position thereof, at a front side of the microphone unit.
Patent document 2 (Japanese Utility Model Laid Open H6-73991) discloses a wind noise preventing type microphone apparatus that has a case in which a microphone unit and a wind noise absorbing laminated body are provided, wherein the laminated body is formed of an acoustic resistance material and two sheets of nonporous hard material which sandwiches the acoustic resistance material therebetween, and each sheet has a small hole made at a position apart from the central part thereof.
According to the microphone apparatuses described in the patent documents 1 and 2, the effect of noise (wind noise) reduction can be obtained. However, the foamed body of the microphone apparatus described in patent document 1 works as an acoustic resistance and the acoustic resistance material in the microphone apparatus described in the patent document 2 works as an acoustic resistance. Accordingly, there is a defect that the speech signal input to the microphone unit attenuates in proportion to the effect of noise reduction and the sensitivity of the microphone unit is significantly reduced.
Both microphone apparatuses described in the patent documents 1 and 2 need a large number of configuration elements, and consequently lowering the cost of production is difficult and the process of manufacturing is complicated. Further, in order to adjust sensitivity corresponding to the kind of microphone unit, plural kinds of foamed bodies or acoustic resistance materials are needed, and the effect of noise reduction will be lost when the sensitivity of the microphone unit is increased by changing a foamed body or acoustic resistance material.